The present invention relates to an air-fuel ratio control technology for setting a feedback control amount to feedback control an air-fuel ratio, while calculating a parameter of a plant model representing a plant between a fuel injection valve and an air-fuel ratio sensor by a transfer function.
Heretofore, in an internal combustion engine, it is common to feedback control an air-fuel ratio to a target value so as to improve the exhaust purification and the fuel efficiency.
There has been disclosed a technique for performing such an air-fuel ratio feedback control with high accuracy (Japanese Unexamined Patent Publication No. 2001-164971), in which a feedback control amount of a fuel injection quantity is calculated by a sliding mode control, while performing a waste time compensation control by a Smith method.
Here, it is possible to adopt a self-tuning control to such a conventional air-fuel ratio feedback control so as to further correspond to a characteristic change of a control object (plant) with high accuracy. In such a case, the feedback control amount is calculated as follows.
A plant model representing a plant between a fuel injection valve and an air-fuel ratio sensor is identified sequentially, to estimate a parameter of the plant model.
Then, using this identified plant model (estimated parameter), the entire system including the plant, a feedback control amount calculating section (in other words, a sliding mode control section) and a waste time compensation control section is represented by one transfer function, and a control gain of the sliding mode control is calculated so that a pole of the transfer function is coincident with a desirable pole from the point of view of response characteristic, overshoot, stabilization period, and so on.
Then, the feedback control amount of the fuel injection quantity is calculated by the sliding mode control utilizing the calculated control gain.
Thus, by adopting the self-tuning method, it becomes possible to perform the air-fuel ratio control corresponding to the characteristic change of the plant with high accuracy. However, there are problems as described below.
(1) The plant can be described by a model as the following equation, for example,
A(zxe2x88x921)y(t)=zxe2x88x92kB(zxe2x88x921)u(t)
A(zxe2x88x921)=1+a1zxe2x88x921+a2zxe2x88x922
B(zxe2x88x921)=b0
where u(t): plant input, y(t): plant output, and k: waste time. Further, since A(zxe2x88x921) is set on the plant output y(t) side, system parameters a1, a2 constituting A(zxe2x88x921) is set as xe2x80x9coutput side parametersxe2x80x9d. Since B(zxe2x88x921) is set on the plant input u(t) side, a system parameter b0 constituting B(zxe2x88x921) is set as xe2x80x9cinput side parameterxe2x80x9d.
The control gain in the self-tuning control is finally calculated by subtracting a calculation value of the input side parameter b0 (or a steady gain B(1) of B(zxe2x88x921) in the case where B(zxe2x88x921) is b0+b1zxe2x88x921+ . . . +bnzxe2x88x92n). Therefore, the smaller an absolute value of the input side parameter b0 (or the steady gain B(1)) becomes, the larger each control gain is calculated.
Consequently, if the calculation value of the input side parameter b0 becomes too smaller, each control gain becomes excessive (that is, the control amount becomes also excessive), to cause a possibility of control divergence. Particularly, in the case where an actual waste time included in the plant is larger than a waste time set for identification, there is caused a problem in that the input side parameter b0 is calculated to be smaller.
(2) The waste time compensation control by the Smith method is for predicting, using the feedback control amount, the plant output after the lapse of waste time, that is, the air-fuel ratio after the lapse of waste time, to determine a control input (plant input). However, since an actual fuel injection quantity (control amount to the fuel injection valve) in the air-fuel ratio feedback control is determined by adding a fixed offset correction amount such as a learning value or various correction values, to the feedback control amount, there may be a possibility that an actual air-fuel ratio is changed due to a variation in the offset correction amount even if the feedback control amount is constant.
Consequently, an error is caused between a predicted air-fuel ratio and the actual air-fuel ratio only by predicting the air-fuel ratio after the lapse of waste time using the feedback control amount, thereby causing a problem in that the waste time compensation control cannot be maintained at high accuracy.
The present invention has been accomplished in view of the above problems and has an object to improve air-fuel ratio control performance in an air-fuel ratio control of an internal combustion engine adopted with a self-tuning control.
According to a first aspect of the present invention to achieve the above object, a basic constitution: to detect an actual air-fuel ratio by an air-fuel ratio sensor; to calculate a fuel injection quantity, using a feedback control amount based on the actual air-fuel ratio and a target air-fuel ratio, at a time when a feedback control is performed; and to inject fuel by a fuel injection valve based on an injection signal corresponding to the fuel injection quantity, to thereby control an air-fuel ratio, is added with a constitution described below:
to sequentially identify a plant model representing a plant between the fuel injection valve and the air-fuel ratio sensor, to calculate an input side parameter (b0 described above corresponds to this parameter) and output side parameters (a1 and a2 described above correspond to these parameters) set, respectively, on an input side and an output side of the plant model;
to calculate a control gain using the calculated parameters (that is, the calculated input side parameter and output side parameters) directly, if an absolute value of the calculated input side parameter is a limit value or above, to calculate the feedback control amount using the control gain; and
to calculate the control gain using the limit value instead of the calculated input side parameter (that is, using the calculated output side parameters and the limit value), if on the other hand, the absolute value of the calculated input side parameter is less than the limit value, to calculate the feedback control amount using the control gain.
A further object of the present invention is to predict an air-fuel ratio after the lapse of waste time with high accuracy, to calculate an accurate feedback control amount, in an air-fuel ratio control apparatus of an internal combustion engine, for calculating an air-fuel ratio feedback control amount while performing a waste time compensation control.
According to a second aspect of the present invention to achieve the above object, a basic constitution: to detect an actual air-fuel ratio by an air-fuel ratio sensor; to calculate a fuel injection quantity, using a feedback control amount based on the actual air-fuel ratio and a target air-fuel ratio, and an offset correction amount to the target air-fuel ratio; and to inject fuel by a fuel injection valve based on an injection signal corresponding to the fuel injection quantity, to thereby control an air-fuel ratio, is added with a constitution described below:
to use a plant model representing a plant between the fuel injection valve and the air-fuel ratio sensor, to calculate a predicted air-fuel ratio after the lapse of waste time included in the plant;
to calculate the feedback control amount, while performing a waste time compensation for eliminating an affect of the waste time using the predicted air-fuel ratio;
to set the offset correction amount; and
to calculate the predicted air-fuel ratio by setting a value obtained by adding the offset correction amount to the feedback control amount as a plant input.
These and other objects and features of the present invention will become understood from the following description with reference to the accompanying drawings.